In Fall 2009, Chicago authorities abandoned a school assignment
mechanism midstream, citing concerns about its vulnerability to
manipulation. Nonetheless, they asked thousands of applicants to
re-rank schools in a new mechanism that is also manipulable. This
paper introduces a method to compare mechanisms by their vulnerability to manipulation. Our methodology formalizes how the old
mechanism is at least as manipulable as any other plausible mechanism, including the new one. A number of similar transitions took place in England after the widely popular Boston mechanism was
ruled illegal in 2007. Our approach provides support for these and
other recent policy changes. (JEL C78, D82, H75, I21, I28)